3 Answers
3

In the expression for the
value of a quantity, the unit symbol
is placed after the numerical value
and a space is left between the
numerical value and the unit symbol.
The only exceptions to this rule are
for the unit symbols for degree,
minute, and second for plane angle
(...) in which case no space is left
between the numerical value and the
unit symbol.

Note that this is what happens if you use the siunitx as intended, namely by using \SI{<value>}{<unit>} to print a quantity:

Note that the quantity is formally a product of the number and the unit, so the space here is showing multiplication.
–
Joseph Wright♦Jun 17 '11 at 6:14

1

@Joseph: Thinking of the quantity as a product of number and unit can be quite useful (e.g. it explains why in tables and graphs, the unit should be separated from the dimension by a forward slash indicating division), but one has to be a bit careful: While different units may be separated by a raised dot to indicate the multiplicative nature, this is not permitted between the number and the units.
–
JakeJun 17 '11 at 7:27

2

That's a typographic convention: in terms of unit analysis these are all products.
–
Joseph Wright♦Jun 17 '11 at 7:38

1

@JosephWright I disagree. It's simply a notational convention. Jake is right. The same way the "dx" in an integral is in ordinary analysis not multiplied with the function term (unless you reinterpret it as a differential to justify the notational abuse in retrospect), the correct way to think of a number-unit cluster is as a pair <number,dimension>. To convince you of this: no number can ever modify a dimension and no dimension can ever modify a number. When you think of "2m" denoting "two meters", this really means "2m = 2 * 1m" (= 2 * <1,m>), not "2 * m".
–
Lover of StructureNov 3 '12 at 5:03

1

@Jake [still addressing Joseph, but I'd like Jake to see this:] There are other constraints: not only does one never write "34·m" (like Jake has pointed out), one also never writes things like "m·1kg"; it must be "1 kg m" or "1 kg·m", and the reason is that there is an underlying constraint (normally obeyed, though I'm sure there are exceptions) that no unit cluster is preceded by nothing. Technically speaking, "(3m²)/(m)" (displayed as an ordinary fraction) is notational abuse as well, because it should parse as "3 (m²)/(m)", but admittedly that is tolerated.
–
Lover of StructureNov 3 '12 at 5:10

My preference would be \;\si\ohm rather than \,\si\ohm or \si\ohm. As Jake mentions, there is a convention, though it strikes me as ill-defined. "A space" is either literal or typographically ambiguous. Definitely some kind of space is preferable to none, but the exact size is likely up to personal taste outside of technical papers (where the behavior is defined, one hopes) or when not using the facilities provided by siunitx.